The conventional method for drilling wells, e.g., oil wells involves driving in rotation a toothed bit fastened to the end of a drillpipe string, the string being generally driven in rotation by a surface installation. A fluid, called drilling fluid or mud, is injected at the level of the bit through the inner space of the pipes. The main purposes of this fluid are to clean the bit and the well by taking the debris up to the surface, to stabilize the walls of the well, to inhibit reaction of the geologic formations in contact with the fluid, etc.
The present invention relates not only to the fluids called drilling fluids, but also to the fluids known as completion fluids, as well as to the fluids called workover fluids, all these fluids being referred to hereafter as well fluids. Completion is an operation which continues the drilling operation when the well reaches the producing formation. Completion notably consists in drilling through the reservoir rock, testing the formation, fitting out the well for production and bringing in. For these operations, the completion fluid may be notably specific to to the reservoir rock and to the effluents produced. Workover operations consist in working in a producing well in order to drill, redrill, clean the well or change equipment.
Well fluids must have characteristics which may be adjusted according to very varied uses, notably characteristic such as viscosity, their density or their filtrate control capacity. In some cases of strongly deflected wells, horizontal wellbores for example, or more generally wells providing considerable friction on the tubular elements lowered in the well, the lubricity of the fluid becomes an important characteristic.
Documents U.S. Pat. Nos. 4,964,615 and 5,318,956 describe the use of esters of vegetable origin in a mixture for drilling fluids. But none of these documents relates to the optimized composition according to the invention.